1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotographic technology.
2. Description of the Related Art
Recently, image forming apparatuses such as copying machines which form images from image data, a facsimile machine or an image scanner used as a computer output device are being more widely used.
Many types of image forming means, including thermal fusion, thermal sublimation, thermal transfer, ink-jet, and electrophotographic, have been developed in response to user demands. Among others, a full-color electrophotographic image forming apparatus using four colors, including cyan, yellow, magenta and black, by arranging four process stations 1a, 1b, 1c and 1d that serve as image forming sections of different colors, as shown in FIG. 18, is now proposed.
In FIG. 18, the process stations 1a to 1d have photosensitive drums 2a to 2d serving as image carriers. After the surfaces of the photosensitive drums 2a to 2d are uniformly charged by primary chargers 3a to 3d, an electrostatic latent image is formed through exposure based on image information from exposure units 4a to 4d, such as LEDs (light emitting diodes) or lasers. Different kinds of toner for the individual colors are imparted to this electrostatic latent image, which is developed by developing units 5a to 5d into toner images.
The individual process stations 1a to 1d serving as process cartridges are detachable from the image forming apparatus main body. The individual process cartridge integrally combines the photosensitive drums 2a to 2d, the primary chargers 3a to 3d, the developing units 5a to 5d, and cleaning means 6a to 6d. 
A transfer medium S serving as a recording medium housed in a paper feed cassette 15 is fed into the image forming apparatus main body by a paper feed roller 16, and conveyed by a resist roller pair 17. Then, the transfer medium S is electrostatically attracted by a transfer conveyor belt 7, serving as a transfer medium carrier, by an attracting roller 12 to which a positive attracting bias voltage is applied by an attracting bias power source 13.
The transfer conveyor belt 7 is attracted and supported by four rollers including a driving roller 8, an attraction opposing roller 9, and tension rollers 10 and 11. The process stations 1a, 1b, 1c and 1d of the individual colors, including cyan, yellow, magenta and black, are arranged substantially perpendicular to the surface of the transfer conveyor belt 7 and sequentially from an upstream side along the moving direction of the transfer conveyor belt 7 (as shown by arrow a).
The transfer medium S, attracted by the transfer conveyor belt 7, passes sequentially through the process stations 1a to 1d of the individual colors. The toner images of the individual colors carried on the photosensitive drums 2a to 2d are electrostatically transferred in sequence. Subsequently, these toner images are heated and pressurized by a fixing unit 18, whereby the toner images are fixed on the transfer medium S to form permanent images.
At this point in time, the transfer medium S, which is a dielectric such as paper or a synthetic resin, carries a large amount of charge since the transfer medium undergoes four runs of the transfer process while passing through the four process stations 1a to 1d. 
As a result, a peeling-discharge phenomenon takes place, at a peeling-separation section, from the transfer conveyor belt 7 provided on the circumference of the driving roller 8, and the toner images, transferred onto the transfer medium S, are disturbed along the discharge pattern.
To solve this problem, it is the usual practice to arrange a neutralizer (neutralizing member) between the transfer conveyor belt 7 and the fixing unit 18, as shown in FIG. 18. For example, a defective image is prevented by arranging a wire-type corona charger 51, as shown in FIGS. 18 and 19, and neutralizing the transfer medium S by means of the corona charger 51 while peeling off the transfer medium S from the transfer conveyor belt 7, thereby preventing peeling discharge. The corona charger 51 includes a conduction shield 53 and a discharge wire 52.
However, use of a neutralizer having the above-mentioned configuration has the following problems.
Use of a wire-type corona charger as a neutralizer is characterized by stabilization of discharge and availability of a neutralizing effect. On the other hand, when cleaning stains off of the discharge wire 52, or when removing transfer medium jam, the discharge wire 52 tends to be easily broken or it is difficult to maintain a sufficient life or ensure safety.
There is available a neutralizing member using a sharp electrode as a neutralizing mechanism posing no risk of breakage of the discharge wire 52, as in a corona charger 51. Use of a sharp electrode can, however, pose other problems.
The neutralizing mechanism using a sharp electrode (needle electrode or the like) has conventionally been arranged near the object of neutralization, since the neutralizing effect becomes more remarkable as the distance between the object of neutralization and the sharp electrode becomes smaller. However, in the neutralizing member using the sharp electrode, the rise in discharge current from the proximity of the discharge threshold value (electric field condition for start of discharge) is steep, making it difficult to stabilize the neutralizing effect because of the unstable discharge.
That is, it has generally been recognized that, if the object of discharge may flap as the transfer medium, a neutralizing member using a sharp electrode did not achieve uniform neutralization. When the distance between the neutralizing member and the object of neutralization is small, an increase in the number of transfers of the object of neutralization in the proximity of the neutralizing member causes adhesion of dust resulting from the object of neutralization itself, to the neutralizing member and an abnormal discharge at this adhesion, thus causing a problem in that the expected neutralizing effect cannot be achieved.